There has been disclosed a technique where a vegetative material is steam-exploded to volume-reduce and solidify the material into, for example, pellets (Patent Literature 1). There has also been disclosed a technique where a biomass solid produced by steam explosion is used as a fuel (Patent Literature 2). There has been further disclosed a technique where a lignin-containing biomass is steam-treated (steam-exploded), thereby facilitating molding (Patent Literature 3). In addition, Non-Patent Literature 1 has described that after steam exploding, hemicellulose in a biomass becomes water-soluble, and Non-Patent Literature 2 has described that when storing a biomass solid fuel after steam explosion, COD (Chemical Oxygen Demand) in discharged water becomes a problem.